This invention relates generally to free electron lasers, and, more particularly, to an electrostatic free electron laser which is electronically tunable over a wide range of frequencies.
One example of a free electron laser is described in U.S. Pat. No. 3,822,410. This type of free electron laser operates on the principle of magnetic bremsstrahlung wherein a periodic magnetic field is utilized to produce radiation. Other types of free electron lasers, more commonly referred to as the Smith-Purcell and Cerenkov lasers are described in a publication by Gover et al entitled "Operation Regimes of Cerenkov-Smith-Purcell Free Electron Lasers and T. W. Amplifiers," Optics Communications, Vol. 26, No. 3, September, 1978, pp. 375-380. In these devices a slow electromagnetic wave structure, periodic waveguide (or grating) or a dielectric waveguide is used to facilitate the interaction of the electron beam and the electromagnetic wave.
However, even with these apparent different physical principles, both the magnetic bremsstrahlung and the Cerenkov-Smith-Purcell lasers have similar gain expressions, similar wave dispersion equations and similar operation regimes. The main difference between the magnetic bremsstrahlung and the Cerenkov-Smith-Purcell lasers is that the magnetic bremsstrahlung laser involves transverse modulation of the electron beam by the transverse periodic force, while the Cerenkov-Smith-Purcell lasers involve direct longitudinal modulation of the electron beam by the longitudinal component of the electric field of the electromagnetic wave. Nevertheless, the interaction between the electron beam and the electomagnetic wave is carried out through longitudinal modulation of the electron beam which is created by the ponderomotive force effect.
Since this is a second or third order effect in the fields, the interaction between the electromagnetic wave and the electron beam in the magnetic bremsstrahlung free electron laser is much weaker than the interaction in the Smith-Purcell-Cerenkov lasers (which are first order effects). Therefore, in principle, one of the differences between the magnetic bremsstrahlung free electron laser and the Smith-Purcell-Cerenkov lasers is that the latter devices can provide higher gain.
Another difference between magnetic bremsstrahlung laser and Smith-Purcell-Cerenkov lasers is in the interaction region width, which affects the power and efficiency of the device. In this aspect the difference is in favor of the magnetic bremsstrahlung free electron laser.
Unfortunately, both types of free electron lasers, as described hereinabove, contain drawbacks which affect both the efficiency and the overall reliability of their operation. In addition to the drawbacks presented above, the above-mentioned lasers also tend to be large and bulky as well as being difficult to tune. Consequently, there remains a void in the area of free electron lasers which needs to be filled.